The market for wireless communications has achieved tremendous growth. Wireless communications offers the potential of reaching virtually every location on the face of the earth. The use of pagers and cellular phones is now commonplace. Wireless communications is also used in personal and business computing. Wireless communications offers networked devices flexibility unavailable using a physically connected network. Untethered from conventional network connections, network users can move almost without restriction. Medical professionals can obtain patient records, real-time vital signs and other reference data at the patient bedside without relying on paper handling or reams of paper charts. Factory floor workers can access part and process specifications without wired network connections, which may be impractical on the factory floor. Warehouse inventories may be carried out and verified using wireless scanners linked to a main database. Multimedia data may be served to various home entertainment devices within a home without a need to install cabling between all of the various home entertainment devices.
Standards for conducting wireless communications between networked devices, such as in a local area network (LAN), are known. The Institute for Electrical and Electronics Engineers (IEEE) offers a standard for multiple carrier communications over wireless LAN systems, IEEE 802.11. IEEE 802.11 includes standard proposals for wireless LAN architectures. Supported architectures include an ad-hoc LAN architecture in which every communicating device on the network is allowed to directly communicate with every other node. In the ad-hoc LAN architecture, there are no fixed nodes on the network and devices may be brought together to form the network “on the fly”. One method of maintaining an ad-hoc network includes defining one device as being a network master with other devices representing network slaves. Another supported architecture is the infrastructure in which the network includes fixed network access points. Mobile devices access the network through the network access points, which may be connected to a wired local network.
IEEE 802.11 also imposes several specifications on parameters of both physical (PHY) and medium access control (MAC) layers of the network. The PHY layer handles the transmission of data between network nodes or devices and is limited by IEEE 802.11a to orthogonal frequency division multiplexing (OFDM). IEEE 802.11a utilizes the bandwidth allocated in the 5 GHz Unlicensed National Information Infrastructure (U-NII) band. Using OFDM, lower-speed subcarriers are combined to create a single high-speed channel. IEEE 802.11a defines a total of 12 non-overlapping 20 MHz channels. Each of the channels is divided into 64 subcarriers, each approximately 312.5 KHz wide. The subcarriers are transmitted in parallel. Receiving devices process individual signals of the subcarriers, each individual signal representing a fraction of the total data.
Other standards also exist within IEEE 802.11. For example, IEEE 802.11b limits the PHY layer to either direct sequence spread spectrum (DSSS), frequency-hopping spread spectrum, or infrared (IR) pulse position modulation. Spread spectrum is a method of transmitting data through radio frequency (RF) communications. Spread spectrum is a means of RF transmission in which the data sequence occupies a bandwidth in excess of the minimum bandwidth necessary to send it. Spectrum spreading is accomplished before transmission through the use of a code that is independent of the data sequence. The same code is used in the receiver (operating in synchronism with a transmitter) to despread the received signal so that an original data sequence may be recovered. In direct sequence spread spectrum modulation, the original data sequence is used to modulate a wide-band code. The wide-band code transforms the narrow band, original data sequence into a noise-like wide-band signal. The wide-band signal then undergoes a form of phase-shift keying (PSK) modulation. In frequency-hopping spread spectrum, the spectrum associated with a data-modulated carrier is widened by changing the carrier frequency in a pseudo-random manner.
Devices are linked through data channels. A data channel is a frequency band used for transmitting data. Multiple carriers within a data channel may be utilized for transmitting data. Carriers are specific frequencies used to provide a set of data. Each carrier is assigned a constellation. The constellation is a map including various points identifying particular symbols used for transmitting a particular set of bits. The number of bits assigned to a point indicates a number of bits transferred per symbol received. Different carriers may be assigned unique constellations.
IEEE 802.11a and IEEE 802.11b specify a specific MAC layer technology, carrier sense multiple access with collision avoidance (CSMA-CA). CSMA is a protocol used to avoid signals colliding and canceling each other out. When a device or node on the network receives data to be transmitted, the node first “listens” to ensure no other node is transmitting. If the communications channel is clear, the node transmits the data. Otherwise, the node chooses a random “back-off factor” that determines an amount of time the node must wait until it is allowed to access the communications channel. The node decrements a “back-off” counter during periods in which the communications channel is clear. Once the “back-off” counter reaches zero, the node is allowed to attempt a channel access.
While communications standards, such as IEEE 802.11a, allow a single transmitting device to provide data to multiple receiving devices, the quality of data received by some receiving devices may be degraded. One quality of a signal is commonly measured using the signal-to-noise ratio of the signal at the receiving device. Another metric to measure the quality of received data is the bit error rate (BER). As the signal-to-noise ratio becomes too low for a particular data signal, the BER associated with a receiving device may be too high for the receiving device. The signal-to-noise ratio of a signal can be affected by the distance the signal must travel. A receiving device may be located too far from a data transmitter. A signal-to-noise ratio can be dependent on the power of the transmitted signal, assuming a sufficient signal to noise ratio may be output by the data transmitter. The transmission power associated with a data signal transmitted to a particular receiving device may be too low. A signal may also be degraded due to interference from other data transmitters or other radio frequency (RF) radiators. A receiving device with a low signal to noise ratio may request data at a lower bit rate from the data transmitter. More transmission time on the data channel can become reserved for transmitting data to the receiving device with the low signal to noise ratio. Accordingly, other devices may not be able to access the data channel as needed. Furthermore, a transmission data rate for a particular data channel may be inadequate for a high-bandwidth receiving device. The data channel can be configured to transmit data at a maximum data rate, such as according to IEEE 802.11 standard or due to a maximum data rate acceptable by a particular receiving device. A high-bandwidth receiving device may require a large amount of data; however, due to limitations configured into the data channel, the required amount of data may not be accessible to the high-bandwidth receiving device using the data channel. From the above discussion, it should become apparent that an improved method of transmitting data to multiple devices is needed.